There is not clear cut answer to that. Casual games like Angry Birds or Abduction (Doodle Jump game) or even simpler games are pretty big due to them catching the largest audience (non-hardcore gamers). However, there are also a couple of titles that are less casual but still successful.

I think what's more important than the genre is the accesibility of a game (how easy is it to pick it up) as well as the marketing that goes along with releasing the game. If people don't know about your game it can be as good as can be, you won't have any players.